Meet the ‘Digester’ — It Turns Your Food Scraps into Energy

Five days a week, a long chrome truck pulls up to the East Bay Municipal Utility District’s wastewater treatment plant. It lifts its hydraulic-powered trailer bed and proceeds to dump 40,000 pounds of what looks like thick sewage into a giant underground mixer. Strangely, it smells … good. The aroma of soil, mint and citrus wafts from the truck. Not the types of scents you typically associate with a sewage plant.

That’s because this type of material isn’t typically found in a sewage plant. It’s food scraps, collected from restaurants from all around the Bay Area. EBMUD is using these food scraps in an innovative program that transforms them to electricity—which in turns powers the sewage treatment plant.

The first program of its kind in the country, EBMUD’s food scraps-to-energy program is on the cutting edge of urban green energy production. It recycles food waste. It helps cut greenhouse gases. It pares down landfills. And EBMUD administrators say this project is just the beginning. With the pilot phase now winding up, EBMUD plans to expand the program, process more food waste, install larger-scale engines and create more electricity.

As the food scraps pour out of the truck, workers hose water into the mixer, which churns, swirls and agitates the concoction. Steam rises from the clumps of dirt, bits of lettuce and other pieces of food. Sophia Skoda, an EBMUD senior civil engineer with a big smile and auburn-colored hair, looks admiringly at the slurry. “It’s already starting to break down,” she says. “It’s starting to compost.”

The pizza crusts, orange peels and leftover fried rice will head from the mixer into a series of pipes, pumps and grinders—getting mashed into a pulp, and eventually ending up in a giant cylindrical vat called a digester. Skoda describes this vat as a “large tank that acts as a stomach,” in which food scraps are kept at a warm temperature and are “digested” by tiny bacteria. As the stomach breaks the mixture down, burping and passing gas, the solids reduce and methane gas rises to the top in a process called anaerobic digestion. That odorless digestive gas is then pumped to EBMUD’s engines to create the electricity that treats the wastewater of the 650,000 people EBMUD serves in the East Bay.

“A lot of other people are looking at this as a model, and we are as well,” says Laura Moreno, an environmental scientist in the U.S. Environmental Protection Agency’s office of pollution prevention and solid waste. The idea of burning methane to create electricity is nothing new. For decades, wastewater treatment plants have been using digesters to covert bio-solids (the treated form of sewage sludge) into methane gas. The big difference with EBMUD is that they are also using leftover restaurant scrapings to do it.

In 2007, three years after EBMUD began the pilot food scraps project, the EPA awarded the agency a $50,000 grant for further research on converting food waste to electricity. “One of their key findings was that food has three times the methane potential of bio-solids,” Moreno says. “Their project has been instrumental nationally. From this food waste that would’ve otherwise ended up in the landfill, we can actually make a good amount of renewable energy.”

After paper, food scraps are the nation’s second largest source of waste. Filling 18 percent of landfills, dinner leftovers, rotting fruits and vegetables, and restaurant refuse make up over 30 million tons of what is sent to dumps each year. When cut off from oxygen, all of this organic matter creates methane. In landfills, where heaps of trash are piled on top of food scraps, a perfect situation is created for generating methane gas. Methane is a huge contributor to global warming—more than 20 times more potent than carbon dioxide.

Right now, EBMUD receives around 30 tons of food waste a day, five days a week. But the agency is looking to increase this amount six-fold. By next spring, EBMUD’s engineers are expecting to ramp up to 200 tons of food waste a day. “Every molecule of methane that you destroy is a greenhouse gas that you have destroyed, that you haven’t let out into the atmosphere,” says Skoda as she stands in front of one of EBMUD’s massive digesters. “Here we capture it. And we are able to harness this power.”

Why Oakland?

In the big open kitchen of the restaurant Flora in downtown Oakland, a dishwasher picks up a plate and scrapes leftover frisee lettuce into a tall gray trashcan. On the other side of the kitchen, a cook prepares food and throws food scraps into another tall gray trashcan. The same goes for the bar. Flora, along with other participating restaurants in Oakland—including Bay Wolf, Pizzaiolo and Camino—meticulously separates food scraps from other waste. It’s this frisee lettuce, along with everything else filling these tall gray trashcans, that will eventually end up creating methane gas in EBMUD’s digesters.

“There’s so much less actual garbage scraps than food scraps,” says Andee Brown, Flora’s general manager, as she points to the big trash bins in the back of the restaurant where all the food scraps from the gray bins are emptied each night. “We started by having just one big green bin, and we found we really needed more compost bins than garbage.” Now the restaurant has one big brown bin for garbage and two green bins for compost. Every day, a company called Recology, which is one of the collection and recycling companies that provides EBMUD with food scraps, picks up these green bins and prepares the contents for digestion.

Oakland has an uncommon recycling system in that commercial recycling is not controlled by the city. “That allows independent recyclers to collect food scraps and other organic materials from commercial businesses outside of any franchise and contract with the city,” says Peter Slote, a city recycling specialist. This means restaurants may opt for Recology to pick up their food scraps to use for digestion, rather than default to the Alameda County Waste Management Authority. “Oakland is a very geographically dense source for that material and that’s different from other cities,” Slote says.

Long before it became a center of upscale boutique restaurants, Oakland had a long industrial history with food. During the 1940’s, 50’s and 60’s, because of cheap rent and proximity to the port and railways, all sorts of food canning, processing and packing companies moved to the city, from Del Monte to Mother’s cookies, making it the largest food-processing center in the country. EBMUD’s wastewater treatment plant was built right at the same time. In order to deal with the large amounts of water from these food factories, EBMUD built up a significant infrastructure, including excess flow tanks, treatment plants and digesters.

But over the course of the last 20 to 25 years, most of those food companies left the East Bay, looking for cheaper land and labor. “Basically, we had all this infrastructure that was built to accommodate customers that essentially left,” Skoda says. With the departure of these companies, EBMUD lost nearly a third of the wastewater it originally calculated it must treat, and the utility stopped using a lot of its infrastructure. But even with reduced wastewater treatment needs, a large portion of the utility’s costs had to stay fixed to keep the plant running and the equipment maintained. “Our choices seemed to be raising rates to customers or figuring out a creative means for this excess capacity,” Skoda says.

The looming digesters sat empty for several years. In 2000, when the California electricity crisis hit, causing rolling blackouts and electricity shortages, EBMUD engineers started thinking about alternative fuels that might run their wastewater treatment plant. They began experimenting with different methods of energy creation, including bio-diesel production from fats, oil, and greases; and working to create methane gas by mixing blood waste from animal slaughterhouses with bio-solids in their digesters.

By 2004, they had also started exploring adding food scraps to the digesters. They began small, taking one truckload a month, and then increased. Not only did the machines seem to work better with the food, they also found that the food increased three-fold the amount of methane they were capturing. It was then that EBMUD’s engineers realized they were on to something big.

How the whole thing works

Underneath the highways that merge onto the Bay Bridge, a dozen digesters sit in a neat row. They are cream-colored, cylindrical and massive. Each can hold up to two million gallons of liquid, sludge or pulp. As the chrome truck dumps its contents into the underground mixing tank, an EBMUD worker looks up at the closest digester and says, “That’s a big gas bag, that’s what it is.”

Before reaching the digester, the food scraps go through an intensive preparing and cleaning process. It starts when they’re picked up from the restaurants. “Whatever you leave on your plate, the chef scrapes into the green bin. We pick it up, we put it in the truck and start the process of cleaning it,” says Malaika Thorne, a development analyst for Recology.

“The grinder is one component,” Thorne says. “It just makes things smaller.” Bones get pulled out, she says, along with any non-biodegradable items, such as eating utensils, aluminum foil and plastic bags. Then EBMUD does its own sorting to get rid of any objects Recology may have missed: chopsticks, seashells, corks, rags, or items that are just too big, like sticks. These items are dangerous because they can ravage the pipes as they are sucked through the process. “We are basically making a milkshake out of it,” engineer Skoda says, explaining that the smaller and cleaner the scraps, the easier it is to pump them quickly to the digester.

After sitting in the digester for a couple of weeks, the original volume of the food scraps is reduced by 90 percent—the great industrial gut has done its job. The gas that arises is pumped over to EBMUD’s Power Generation Station. There, three internal combustion engines produce 2.2 megawatts of electricity each. Just one of these megawatts provides enough energy to power up to 1,400 homes per day. “That electricity we use when we need it on our own plant site, to pump things around, keep the lights on and do what we need to do to get water clean before it is sent out to the bay,” Skoda says.

Treating wastewater uses a tremendous amount of electricity; in the U.S., three percent of all electricity is used in water and wastewater treatment. On average EBMUD’s plant uses 4.5 megawatts per day. When lots of water comes into the plant—during a rainy winter, for example—they must purchase electricity. But there are also times, even during a wintry day, when water use dips temporarily and they produce excess electricity. Then they’re able to sell it back to the power grid. “And so we are selling to the grid at certain times of day,” says Skoda. “And other times of day we are needing that electricity and using it.”

Overall, EBMUD now produces 90 percent of all of the electricity it uses and is only buying 10 percent, whereas before using food scraps the agency was only producing 40 to 50 percent. Still, for EBMUD, this is not enough. The utility’s engineers want it to be completely self-sufficient. Right now, they are in the process of installing a new turbine engine that will generate 4 megawatts of electricity, which is almost double what they’re generating right now. By next spring, the utility plans to be permanently selling more electricity than they buy.

What’s next?

EBMUD’s Power Generation Station is a giant building with raised ceilings to house the combustion engines. Construction workers hurry around, operating lifts, laying foundation. The chugging noise from the engines is deafening. Behind the building is a huge structure hidden underneath a cloak—the new turbine engine EBMUD is installing.

This experiment of creating electricity with food scraps is something that the EPA has watched closely. “If 50 percent of the food waste generated each year in the U.S. was anaerobically digested,” says Moreno, the EPA environmental scientist, “enough electricity would be generated to power over 2.5 million homes a year.”

Based on EBMUD’s model, the EPA has just released free software called the “co-digestion economic analysis tool,” which helps wastewater treatment plants decide if it’s economically possible to mimic a program like EBMUD’s. “We are hoping that it will provide cities with that initial look,” Moreno says, “as they either try to reduce waste or reduce their climate footprint.”

Without existing infrastructure, she says, these digestion processes are expensive for cities to build. But it’s common, Moreno says, for wastewater treatment plants to have extra digesters and pumping equipment. In California, for example, there are about 140 wastewater treatment plants; 15 to 30 percent of them have the extra machinery necessary for this type of project.

At EBMUD, there is still plenty room to grow. Right now, of the 12 digesters on site, only six are being used. Once the utility has the new turbine engine installed, the only thing it will need is more food to create the gas to keep this engine fed. EBMUD already gets food scraps from parts of San Francisco, Alameda and Contra Costa counties. It is looking to partner with more collection companies and has had talks with the cities of Berkeley and Fremont to obtain their food scraps.

Ideally, EBMUD would like to get food scraps from residential homes too, which would mean teaching people how to carefully sort what can and can’t be digested and putting together a system of collecting the scraps. The agency would also like for every restaurant in Oakland to choose Recology or any other EBMUD partner to collect the food scraps, which would then contribute to more food destined for the digesters.

Once EBMUD has more food, it can become even greener. Right now, EBMUD mixes the food scraps with bio-solids from its wastewater treatment plant. That means none of what’s leftover after digestion can be composted in food crops, since bio-solids are essentially sewage sludge and cannot be used as clean organic compost. Instead, part of the mixture is sent to a landfill to be “alternative daily cover”—material put on top of a landfill to control disease, fire, odor, blowing trash, and scavenging—and the other part is used for land application in non-food crops, such as oils, fibers and animal feed.

But if the digesters were filled with only food scraps, what was left over could be used as clean compost. That would just add to EBMUD’s already good project, explains Peter Slote, the City of Oakland’s recycling specialist. “Ultimately we want to use the post-energy solids that are left over after you’ve recovered the energy from them,” he says. “Then anaerobic digestion just becomes something I call, ‘a funny thing happened on the way to the compost pile.’”

EBMUD is working on getting a food-only digester up and running by 2012. The utility is partnering with Recology for this project, and is also building an on-site grinder and pre-processing facility. This would cut down on Recology’s transportation costs and let the company do the grinding and sorting right next door.

EBMUD shift supervisor Joe Augustine, who runs the area of the plant that handles the food waste, has been working in wastewater treatment for 25 years and says the full circle of turning food scraps to energy and then to compost—enormous machines mirroring the full wonder of human digestion—is the wave of the future. “I just want to see more food,” he says, as he scans EBMUD’s sprawling facility. “Bring it on.”